Purging Calculations - Liquid

Find flow rate through the blow-off valve by using the formula for critical velocity,
Method A – Finding the flow rate through the blow off valve(s) using a modified Weymouth formula for critical velocity. Method B where pipe section is assumed to be filled with air prior to purge.
Blow off pressure should be between 18-20 psi and maintained. Blow off coefficient of valve must be known.

𝑄 = 𝐾(𝑃2)
where
𝑄 − Flow Rate[MSCFH]
𝐾 − Blowoff coefficient for standard sizes[MSCF/(h x psia)]
𝑃2 − Pressure just upstream of blowoff valve(recommended 18−20)[psia]

PTLB Testing 2.6_Intro.png

From rearranged Weymouth formula to find an estimate pressure value of necessary to maintain this flow rate:

PTLB Testing 2.6_Intro 2.png

𝐿 − Length of Purge Section[mi]
𝐷 − Internal Pipe Diameter[in]
𝑃1 − Purge Gas Injection Pressure[psia]

PTLB Testing 2.6_Intro 3.png

𝑃𝑀−Average Pressure[psia]

Method A

Recommended purge time is 2T. The minimum purge time in minute is

PTLB Testing 2.6_Intro 4.png

𝑇𝑎 − Minimum Purge Time[min]
𝑇𝑟𝑎 − Recommended Minimum Purge Time[min]

PTLB Testing 2.6_Intro 5.png

Where
𝑉𝑎 − Volume of Gas Lost − min time[MSCF]
𝑉1 − Actual Volume of Pipe Section−assume filled with air before purge[MCF].
𝑉𝑟𝑎 − Volume of Gas Lost−recommended time[MSCF]

Method B

Recommended purge time is 2T. The minimum purge time in minute is

PTLB Testing 2.6_Intro 6.png

𝑇𝑏 − Minimum Purge Time[min]
𝑇𝑟𝑏 − Recommended Minimum Purge Time[min]
PTLB Testing 2.6_Intro 8.png

PTLB Testing 2.6_Intro 7.png

Where
𝑉𝑏 − Volume of Gas Lost−,min-time[MSCF]
𝑉1 − Actual Volume of Pipe Section−assume filled with air before purge[MCF]
𝑉𝑟𝑏 − Volume of Gas Lost−recommended time[MSCF]

Input Parameters
To create a new case, click the “Add Case” button
Select the Purging Calculation application from the Pipeline Testing module.
Enter Case Name, Location, Date and any necessary notes.
Fill out all required fields.
Make sure the values you are inputting are in the correct units.
Click the CALCULATE button.
Blow-off Pressure, just Upstream of the Blow-off Valve (Recommended 18-20) (psi)
For standard blow-off size(in) (dropdown)
Internal Diameter of Pipe Section(in)
Length of Pipe Section(ft) (dropdown)
PTLB Testing – Liquid 2.4 Input.png

Outputs/Reports
View the results.
If an input parameter needs to be edited be sure to hit the CALCULATE button after the change.
To SAVE, fill out all required case details then click the SAVE button.
To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.
To generate a REPORT, click the REPORT button.
The user may export the Case/Report by clicking the Export to Excel/PowerPoint icon.
To delete a case, click the DELETE icon near the top of the widget.
Flowrate at Critical Velocity (MSCF)
Purge Gas Injection Pressure(psi)
Actual Volume of Pipe Section (assume filled with air before purging)(MCF)
PTLB Testing – Liquid 2.4 Output.png

Related Links
Pipeline Testing – Liquid Services